Outdoor or indoor systems?

To answer this question, we first need to see what successful producers are doing elsewhere.

Harvesting fish from a large earth pond using seine nets.
Photo: Nicolas James

In tropical Asia, tilapia are generally produced in ponds. These can vary in size from 1 000m² to a hectare (above this they become difficult to manage). In Egypt, the world’s second largest tilapia producer, ponds are partially covered with tunnels to elevate water temperatures to the 26°C to 32°C preferred by the species.

In cooler climates, re-circulating systems are used either indoors, as in the UK, or in tunnel-based re-circulating and filtered systems, as in South Africa, where our high-altitude, or high-latitude winters are too cool otherwise for tilapia survival. The ideal is to have water temperatures as near the optimum for as long as possible. However, this does not rule out tilapia culture if this isn’t physically or financially achievable. Fingerlings stocked in spring can grow to market size by autumn and then harvested.

But a rotational harvest throughout the year is preferable, with smaller quantities of fish harvested at regular intervals to ensure the customer receives a constant, reliable supply. Whatever the case, it’s a fallacy to say that, because SA’s water temperatures do not stay at ‘optimum’ for the fish year-round, farming them is uneconomic.

That’s akin to saying that maize or any other seasonal crop should be harvested all year round. And the impressive tilapia production by temperate countries such as Egypt (which has the same temperatures as Polokwane in Limpopo) proves the point.

Requirements
Pond aquaculture is demanding in terms of space and water, while re-circulating aquaculture systems (RAS) in tunnels are more intensive, but more demanding in terms of system design and operation. For pond aquaculture to be viable, a production of around 6t/ha to 10t/ ha is the goal, whereas in tunnel tilapia culture, a 30m to 50m tunnel should produce 5t/year to 20t/year if designed and run correctly.


Red tilapia are vulnerable to predation in pond systems so it is preferable to use a tunnel system.

What are the requirements for warm-water pond aquaculture? Clearly the first essential is an adequate supply of good quality water. For tilapia, saline or brackish water will do, as their salinity tolerance is extremely high, especially our local Oreochromis mossambicus. Gentle slopes are preferable and totally flat or steeply sloping ground, less so. You must be able to drain the ponds, which is why so many farm dams are unsuitable for tilapia farming.

A depth of 1m to 2m is the best. Deeper water doesn’t warm up adequately and is often poor in oxygen and therefore unsuitable for fish culture. Soil water-retention is also important, so sandy soils are out and clay-based soils preferred. Anyone with access to a water flow of 0,01 cumecs (10l/second) or more, could consider pond aquaculture if the soil is sufficiently water-retentive.

Water can be re-used for irrigation – the slight enrichment can only help plant growth. If less water is available, then RAS becomes an option. If land space is also limited, then a re-circulating unit in one or more 30m to 50m hothouse tunnels is worth considering. Tunnel systems can be built in the most unlikely places (ours are on terraced hillsides) and require very little space. Essentials here are protection from wind and, in hail-prone areas, a covering of hail-netting. Tunnel plastic can last as long as 10 years in well-built structures in sheltered locations.

Intensive
Tunnel aquaculture is more intensive than pond farming, and demands a greater technical skill, both in design and operation. However, greater yields can be realised as there’s a high degree of control over all aspects of the operation.
Furthermore, water demand is low as most of it is filtered through a filtration unit enclosed within the tunnel, together with the grow-out tanks to retain warmth. Boreholes are usually adequate, or a water piped from a small dam. Obviously, consideration must be given to avoiding the likelihood of contamination by pesticides, weedkillers or tick sprays, which are all lethal to fish.

Nicholas James is an ichthyologist and hatchery owner. Contact him at [email protected]. Please state ‘Aquaculture’ in the subject line of your email.